linux-kernel - copy_from_user_*() and buffer zeroing

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Date:	Tue, 26 Nov 2013 13:07:07 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...nel.org>, Al Viro <viro@...IV.linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
CC:	Vitaly Mayatskikh <v.mayatskih@...il.com>
Subject: copy_from_user_*() and buffer zeroing

I just started looking into the horribly confused state of buffer
zeroing for the various copy_from_user variants.  This came up after we
did some minor tuning last week.

copy_from_user_inatomic() seems to be documented to not zero the buffer.
 This is definitely *NOT* true on x86-64, although it does seem to be
true on i386 -- on x86-64, we carry along a "zerorest" flag but in all
possible codepaths it will be set to true unless the remaining byte
count is zero anyway.

Furthermore, on at least x86-64, if we do an early bailout, we don't
zero the entire buffer in the case of a hard-coded 10- or 16-byte buffer
(why only those sizes is anybody's guess.)  See lines 71-88 of uaccess_64.h.

I'd like to figure out what is the required and what is the desirable
behavior here, and then fix the code accordingly.

	-hpa
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